_slides_pmi285-wu-2-jan_04 - Oxidative Stress oxidants...

Oxidative Stress
oxidants
antioxidants
Reactive oxygen species
Reactive nitrogen species
Free radicals
Vitamin C, E
Carotenoids, Se
Glutathione, Thioredoxin
Specific enzymes: SOD, Catalase

Oxidative Stress
• What are oxygen radicals or reactive
oxygen species?
• How reactive are reactive oxygen species?
• How are reactive oxygen species generated
in the cell?
• How do cells protect themselves against
reactive oxygen species
?

Oxygen Radicals and Reactive Oxygen
Species
• O
2
, O
3
• Superoxide anion, O
2
-
.
• Hydrogen peroxide
• Hydroxyl Radical
• Hypochlorous acid, HOCl
• Aldehyde
• NO, NO
2
, Peroxynitrite
• Lipid alkoxyl radicals
• Lipid peroxyl radicals

Reactive Oxygen Species Half-life
O
2
O
3
H
2
O
2
O
2
-.
.
OH
RO
.
ROO
.
NO
.
ONOO
-
oxygen (in water)
ozone
hydrogen peroxide
superoxide anion
hydroxy radical
alkoxyl radical
peroxyl radical
nitric oxide
peroxynitrite
10
-6
sec
stable (not occur in vivo)
stable
spontaneous dismutation
10 -9 sec
10
-6
sec
7 sec
5.6 sec (0.1 sec in heart)
1-2 sec

Free Radicals: Definition
A free radical is defined as any species that contains one or more
unpaired electron occupying an atomic or molecular orbital by itself
.
e.g. O
2
Box diagram
Lewis dot diagram

Free Radical: Chain Reaction
Unpaired electron can serve as an electron
donor (nucleophilic)or as a receiver.
+ R
C
(R-C)
R + C
R + C
R-C
+
C R
(electrophilic)
+ R
R
C
(nucleophilic)
+
C
(Note: Transition metals, such as Fe and Cu, can also serve as
an electron donor or as a receiver)

Lipid Peroxidation
•Any free radical generated close to membrane can attack the
unsaturated fatty acid side chains of the phospholipid of membrane.
•Transition metal will accelerate this reaction.
R--C -H + HO R--C + H
2
O
carbon-centered radical
R--C - OO
peroxyl radical
R--C + O
2
R--C -OO + R’-H R--C- OOH + R’
lipid hydroperoxide
R--CHO
(e.g. malondialdehyde, 4-hydroxynonenal)
R--C-OOH
CH
3
(CH
2
)--CH--C = C -CHO + Protein-SH
OH
H
CH
3
(CH
2
)--CH--CH - CH
2
-CHOH
O
H
4-hydroxynonenal
S-protein
protein CH
NH - CH
N = CH
protein-NHCH=CH-CH=N-protein (Intermolecular cross-link)
CHO--CH
2
--CHO + NH
2
--protein--NH
2
(Intramolecular cross-link)
Malondualdehyde + 2 Protein-SH

Transition Metal Ion in Lipid Peroxidation
R - OOH + Fe
+2
R - O + Fe
+3
+ OH
-
alkoxyl radical
R - OOH + Fe +3 R - OO + Fe +2 + H +
peroxyl radical
R - O + R’-H R - OH + R’
R - OO + R”-H R -OOH + R”
R’ + O
2
R’ -OO

Prooxidant Effect
•Most antioxidants are capable to generate oxidants through
the chemical nature of their redox cycle.
•Free radical molecule can be generated from antioxidant
through the catalysis of a transition metal.
O - C = C -OH + Fe
+2
+ H
+ HO -C = C -OH + Fe
+3
Normally:
2 O - C = C - OH + 2H+ O = C - C = O + HO - C = C - OH
When oxygenation:
2 O - C = C - OH + O
2
O = C - C = O + H
2
O
2
(dehydro- )

One-electron Reduction Potential
2.4
1.6
0.8
0.0
Reduction Potential (E
0
/Volts)
HO , H
+
/H
2
O
RO , H
+
/ROH
ONOOH/ NO
2
ROO , H
+
/ROOH
NO
2
/NO
2
-
GS /GS
-
Ascorbate
-
, H
+
/ascorbate monoanion
Fe
+3
-EDTA/Fe
+2
-EDTA

Formation of Oxidants
Electron-Transfer Reactions
O
2
e
-
e - e - e -
O
2
-
HO H
2
O
H
2
O
2
0
H
Hydroxyl radical
Superoxide anion

Superoxide Anion Radical
Two reactions are important in a cellular setting:
•